1. Field of the Invention
The present invention relates to an outboard engine assembly including an internal combustion engine having a vertical crankshaft.
2. Description of the Prior Art
Internal combustion engines operating on four-stroke cycle for use as outboard engines are advantageous from the standpoints of fuel economy and emission control because they are free of a wasteful discharge of air-fuel mixture which would otherwise occur with two-stroke internal combustion engines.
Outboard engines for use on motorboats should preferably be compact, particularly with respect to height and width, to minimize the engine mass that projects into the motorboat when the engine is tilted up, especially where the engine is mounted on the stern of the motorboat with a partition wall, or to avoid physical interference between two engines mounted on the motorboat when the motorboat is steered. Such a requirement is also to be met by four-stroke outboard engines.
Outboard engines in the form of four-stroke outboard engines of 45 hp have already been put to use. Some outboard engines that are designed for a compact configuration comprise four-stroke outboard engines with vertical crankshafts and three cylinders arranged in line.
Higher engine output power may be achieved by outboard engines with four or more cylinders, which may be arranged in a V shape to meet height and width requirements. A V-shaped four-stroke outboard engine with six cylinders is known from Japanese laid-open patent publication No. 62-267561. The six-cylinder outboard engine is however considerably heavy and large due to an increased number of parts used.
Outboard engines are also required to transmit less vibration to motorboat hulls on which they are mounted. One conventional vibroisolating structure which supports an outboard engine comprises a case having with large recesses defined in a side wall thereof, and rubber mounts fitted in the respective recesses, the outboard engine being supported by the rubber mounts. The case is however relatively low in rigidity, tending to resonate with the engine. The larger the recesses for receiving larger mounts, the lower the rigidity of the case, resulting in greater risk of resonation with the engine.
An outboard engine assembly generally comprises an engine, a vertical shaft coupled to and extending downwardly from the engine, a propeller shaft coupled to the vertical shaft and having a propeller, and a case housing the engine, the vertical shaft, and the propeller shaft. The outboard engine assembly is supported on the stern of a motorboat by an attachment such as a bracket. Thrust forces produced by the propeller are transmitted through the case and the attachment to the hull of the motorboat.
If the engine is larger in size for producing higher engine output power, then the engine is heavier, making it necessary to strengthen the arrangements for supporting the outboard engine assembly and transmitting thrust forces.
Vibration transmitted from the engine to the hull may be attenuated by a resilient vibroisolating body as a rubber mount interposed between the case and the attachment. Since the weight of the outboard engine assembly is imposed on and the thrust forces are applied to the rubber mount, the rubber mount should be harder in the direction in which the thrust forces are applied and softer in all other directions for absorbing applied vibrations. One known such resilient vibroisolating body is disclosed in U.S. Pat. No. 3,599,594. The disclosed resilient vibroisolating body is interposed between a rigid body and an engine case, and is of such a uniform property that its resilient characteristic varies at a uniform rate in the direction in which the thrust forces are applied. Stated otherwise, the resilient vibroisolating body fails to have a harder property desirable when larger thrust forces are applied, a softer property desirable when smaller thrust forces are applied, and a transient property between the harder and softer properties, all in one system.
Certain outboard engine assemblies have a rubber mount comprising a core and a resilient member disposed around the core. The rubber mount together with a cover, which is held against an engine, is fastened downwardly to an engine attachment by a bolt. A space is needed between the engine and the engine attachment for accommodating the rubber mount, and an additional space is also required to house the head of the bolt. If the head of the bolt is to lie flush with the rubber mount, then the region of the cover which receives the head of the bolt has to be reduced in thickness. However, since a clearance is needed between the rubber mount and the cover for the insertion of a fastening tool, the position of the bolt has to be shifted outwardly by a distance corresponding to the clearance. Furthermore, the cover is disposed in a gasket of the engine, and should be designed with sufficient considerations for supporting the outboard engine assembly.
An outboard engine assembly with a vertical shaft is mounted on the stern of a boat hull such that the cylinder axes extend substantially horizontally, the crankcase is positioned closely to the boat hull, and the cylinder head is positioned remotely from the boat hull. To support the outboard engine assembly on the boat hull for isolating engine vibration, it is effective to locate upper mounts for the engine rearwardly of the vertical shaft and support the upper mounts substantially in alignment with a torque roll axis. The upper mounts should preferably be arranged in a closed loop to achieve a sufficient mount frame rigidity against vertical shock loads. One example of such closed-loop configuration is disclosed in U.S. Pat. No. 3,599,594.
Generally, a four-stroke outboard engine assembly has an oil pan positioned below the cylinder block of the engine, so that lubricating oil returns downwardly into the oil pan. U.S. Pat. No. 3,599,594 shows a two-stroke outboard engine, and discloses no lubricating oil system applicable to a four-stroke outboard engine. In a V-shaped four-stroke outboard engine with four cylinders, if the cylinders are ignited at equal intervals, then reactive forces produced by the drive torque of the engine are relatively small, but a primary inertial couple is relatively large. If the cylinders are ignited at unequal intervals and the crankpins are angularly spaced 180.degree. from each other, then a primary inertial couple is reduced, but reactive forces produced by the drive torque of the engine are relatively large. Therefore, large vibrations are transmitted from the outboard engine to the boat hull on which it is mounted.